Creatinine is a break-down product of creatine phosphate in muscle, and is usually produced at a fairly constant rate by the body, depending on muscle mass. Creatinine is chiefly filtered out of the blood by the kidneys, though a small amount is actively secreted by the kidneys into the urine. This test is most widely used to measure renal function, also to calculate the estimated glomerular filtration rate, which is used as a screening test to look for evidence of kidney damage. Creatinine test may be ordered when a CT scan is planned, prior to and during certain drug therapies, and before and after dialysis to monitor the effectiveness of treatments. INCREASED creatinine levels are associated with glomerulonephritis caused by infection or autoimmune disease, bacterial infection of the kidneys, acute tubular necrosis, urinary tract obstruction caused by prostate disease or kidney stone, congestive heart failure, atherosclerosis, or complications of diabetes. LOW blood levels of creatinine are not common, but they are also not usually a cause for concern. They can be seen with conditions that result in decreased muscle mass. See also Creatinine Clearance, Serum & 24 Hrs Urine, Blood Urea Nitrogen, Serum and Cystatin C, Serum